For example, gases used for a semiconductor process include numbers of corrosive gases (such as BCl2, Cl2, HCl, and ClF3) and reactive gases (SiH4 and B2H6), and in the case of measuring the flow rate of such gas, a thermal flowmeter that can indirectly measure the flow rate without the need to bring a sensor terminal or the like into direct contact with the gas may be used.
The thermal flowmeter is one including: a fluid resistor that is arranged in a main flow path through which the gas flows; a narrow tube that connects between the upstream and downstream sides of the laminar flow element as a bypass and forms a sensor flow path through which the gas flows at a flow rate having a predetermined ratio with respect to the main flow path; and two electrical resistive elements that are provided on the upstream and downstream sides on the outer surface of the narrow tube. In addition, applied voltages are controlled so as to make the temperatures of the upstream and downstream side electrical resistive elements constant, or make currents flowing through these electrical resistive elements constant. Further, on the basis of the voltages applied in accordance with such constant temperature control or constant current control, a sensor output calculated value is calculated, and on the basis of the sensor output calculated value, and flow rate calibration curve data indicating the relationship between a sensor output calculated value and a flow rate, a flow rate corresponding to the sensor output calculated value is outputted.
Meanwhile, in the case where the type of gas to be used is not determined at the time of factory shipment, not the above-described corrosive or reactive gas, but inert gas (such as N2) is flowed through a flow path, and on the basis of an actual flow rate of the inert gas and a corresponding sensor output calculated value, flow rate calibration curve data is calibrated. Accordingly, when the thermal flowmeter storing the flow rate calibration curve data calibrated using the inert gas measures the flow rate of corrosive gas or reactive gas without any correction, an actual flow rate is not correctly outputted. For this reason, when measuring the flow rate of gas of a type different from the gas used for the calibration, an outputted flow rate is corrected using a conversion factor that is a ratio between a flow rate of the gas used for the calibration and a flow rate of the gas currently flowing at each sensor output calculated value.
It is known that the conversion factor does not have a fixed value with respect to a flow rate, but as a flow rate increases, changes at an increase rate determined for each gas type. For example, in Patent Literature 1, a conversion factor is approximated as a cubic function of a flow rate, and the flow rate function representing the conversion factor is preliminarily stored for each gas type. Further, it is configured that on the basis of flow rate calibration curve data used for calibration and the flow rate function representing the conversion factor of that gas type, flow rate calibration curve data on the gas type is calculated, and an appropriate flow rate is outputted for any gas type.
However, in order to make it possible to calculate flow rate calibration curve data on each gas type in the configuration of a thermal flowmeter in Patent Literature 1, a flow rate function representing a conversion factor of that gas type should be identified in advance by experiment. Since a large number of gas types are present for use in a semiconductor manufacturing process, it takes time very much to identify flow rate functions representing conversion factors of all of possibly-used gas types.
Also, the conventional thermal flowmeter as described in Patent Literature 1 cannot automatically distinguish a gas type flowing through a flow path, and therefore every time a gas type to be flowed through the flow path is changed, a user is required to manually change settings such that a flow rate function representing a conversion factor of a corresponding gas type is used. In the semiconductor manufacturing process, in some cases, thermal flowmeters are respectively provided in multiple flow paths, and gas types flowing through the respective flow paths are different, so that it is very complicated for a user to make setting changes as described above to the respective thermal flow meters. For this reason, even in the case where the thermal flowmeter is configured to be able to correct the flow rate calibration data for each gas type, it is difficult to use the thermal flowmeter in an actual use environment.